Short version:

Long Version

Using Wireshark, I discovered that no response packets are sent for any request with a very long URL (somewhere between 900-1100 characters, the length of the URL seems to vary based on browser or some other factors). The problem can be reproduced independently of stackoverflow by simply doing a Google search for a very long string (e.g. http://www.google.com/search?q=asdfasdf...[many more characters]...asdfasdf). However, if Google secure search (https) is used, the problem doesn't occur. This seems to indicate that something (possibly a firewall, router, or, more likely, Websense) is inspecting packets and rejecting ones that contain long URLs. The URLs of https traffic can't be inspected because the packet is encrypted, so the packets aren't filtered.

As long as you still have the cookie containing the authentication information, you won't see the problem, but if you log out, you won't be able to log in again. I noticed the problem because I was already logged into stackoverflow.com when my company activated Websense filtering, but I wasn't logged into meta.stackoverflow.com because I don't visit that site very often. When I tried to log in, I saw the blank page even though I was able to continue to use stackoverflow.com normally.

I had the same problem for a long time and it was related to Websense. I sent this article to our security admin and he configured the Cisco router to stop truncating the long URLs, which was causing the OpenID authentication to fail.

%URLF-6-URL_BLOCKED: Access denied URL ; client server
This message is logged for each URL requested that is blocked by Websense software. The message includes the blocked URL, the source IP address/port number, and the destination IP address/port number. Long URLs are truncated to 300 bytes and then logged.